For example, finished dimensions, the level of geometric accuracy, and the like for a cylinder or a fuel injection nozzle of an automobile engine greatly influence power performance and a fuel consumption efficiency of an automobile. For testing, a contact-type measurement machine such as a roundness measurement machine or a surface roughness meter has been generally used. However, in recent years, an optical non-contact-type measurement machine has been introduced for the purpose of not damaging a measurement target object.
A unit configured to obtain shape data on an inner surface of the measurement target object in a non-contact state employs, for example, the image diagnosis technique (the optical imaging technique) of three-dimensionally emitting laser light and capturing interfering light from reflected light, thereby performing numerical processing for a three-dimensional shape by a general method such as a heterodyne method to measure the geometric accuracy.
In a medical field, a method for providing an observable tomographic image of an affected area of an inner portion of a human body, such as X-ray CT, magnetic resonance imaging, and an optical coherence tomography (OCT) image for emitting far-red light with excellent permeability and capturing reflected light to fetch, utilizing coherency of light, numerical data on a three-dimensional shape, has been studied and utilized.
A representative structure of an observation device employing the technique of irradiating an inner peripheral surface of a mechanical device or a mechanical component with a ray of light to observe or measure the inner surface is as described in Patent Literatures 1 to 3, for example.
In an OCT endoscope described in Patent Literature 1, rotation force of a motor is, as illustrated in FIG. 8 of this literature, transmitted to a rotary shaft through a belt, and is further transmitted to a lens unit through a flexible shaft including, e.g., an optical fiber passing through the inside of a tube-shaped optical sheath. However, in this configuration, a two-dimensional tomographic image illustrated in FIG. 26 of this literature can be obtained, but no three-dimensional image can be obtained.
An OCT endoscope described in Patent Literature 2 employs an OCT three-dimensional image system. In this system, an elongated tube-shaped catheter is inserted into an annular guide catheter illustrated in FIG. 1 of this literature, and the catheter includes an optically-connected rotatable and slidable optical fiber or core. A body tissue is irradiated in such a manner that the optical fiber is rotated while being moved in a length direction as illustrated in FIG. 3 of this literature, and an analysis image is observed. However, in this configuration, there is a problem that abrasion powder is caused due to friction between an inner peripheral surface of the catheter and an outer peripheral surface of a drive shaft. Moreover, due to friction, warpage, torsion, and the like of the drive shaft, an uneven rotation speed, a delay in rotation transmission, torque loss fluctuation, and the like are caused. For these reasons, the resultant analysis image is distorted, and required accuracy for spatial resolution and definition cannot be obtained.
A technique described in Patent Literature 3 employs an endoscope. In the endoscope, a motor (7, 8) respectively rotates a sheath tube (5, 6) and a wedge prism (3, 4). In this manner, an emission light ray direction is changed forward so that a portion to be observed upon intravital observation can be freely changed.
However, in this configuration, the rotation frequencies of the two motors are not synchronized with each other, and no rotation detection pulse from the motors is utilized. For these reasons, only a change in the light ray direction can be performed. Thus, a ray of light cannot be emitted in a spiral pattern. For this reason, a three-dimensional image cannot be obtained by capturing and calculation of reflected light from the front by a computer. Moreover, the sheath tube is rotatable, but rotation backlash or play (e.g., 10 microns) in an axial direction (a longitudinal direction) is allowed. For this reason, the sheath tube rotates while vibrating in the axial direction. A distance between a sensor unit and a testing target is changed by the distance of the backlash or play. For this reason, an accurate distance to the testing target cannot be measured, and image definition is low.